Small strongyles

Pathogenesis - larval stages

The larvae of most cyathostome species penetrate the gut wall as far as the mucosa but no further, but a few species will go as far as the muscularis mucosa. However deep the penetration, all larvae become encapsulated in small nodules in which they continue their development. These nodules are readily visible at necropsy and may be quite numerous   - Poynter has  recorded as many as 60 nodules per square centimeter of mucosa in work published in 1970 and others have recorded similar numbers. However, with the use of more effective anthelmintics in parasite control programs far fewer nodules have been  seen at necropsies in more recent times.

The size and location of nodules appears to depend on the species involved. For example Cylicostephanus species appear to develop in the mucosa producing small red to blackish nodules 2-3mm in diameter. Cylicocyclus  and Gyalocephalus species penetrate to the submucosa and provoke larger nodules anywhere from 3-6mm in diameter.


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Encapsulated small strongyle larvae in the cecal wall as seen at necropsy.
Image courtesy of Professor Max Murray
Encapsulated small strongyle larvae shown via trans-mucosal illumination.
Image courtesy of Dr. J.H. Drudge and Hoechst Roussel.

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A close-up view of encapsulated small strongyle larvae in the cecal wall.
Image courtesy of Merial Inc
Histological section of the large intestine showing an encapsulated small strongyle larva in the mucosa
Image courtesy of Dr A. O. Mathison


Pathological changes tend to be more severe in equines less than two years old. In these animals, heavy infections tend to produce  gross changes in the cecum and colon that are characterized by hemorrhagic, catarrhal and fibrinous enteritis. The mucosa tends to be thickened and edematous with varying numbers of ulcers produced by larvae escaping from their nodules.  The color of the mucosa varies from a normal greyish pink with varying numbers of small hemorrhages to a dark red in severe infections. Mucus production tends to be significant and some areas of the mucosa may have fibrinous deposits.

Pathogenesis - adult stages

Pathological changes due to adult small strongyles tend to be less severe than   those due to either their larval stages or adult large strongyles. Their shallow buccal capsules mean that they feed mainly by grazing on  surface areas of the mucosa in contrast to the larger bites taken by the large strongyles. However,  small strongyles tend to be present in much greater numbers than large strongyles so their cumulative effects can be more serious especially where parasite control programs are poorly implemented or ineffective. However, larval migrations of Strongylus vulgaris are potentially more pathogenic because of their migrations through the mesenteric arterial system.



Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000